Sunday could be a pivotal day in the decision-making process for the Keystone XL Pipeline in the United States.

Tens of thousands of people are expected to gather at the Mall in Washington, D.C., to protest the idea of a building a pipeline route from Alberta’s oilsands to Gulf Coast refineries in the U.S. south.

Activists say at stake is not only the pipeline but how accountable U.S. President Barack Obama is when it comes to his pledge to reduce greenhouse gas emissions.

The protest will take place in plain view of the White House’s Oval Office, and according to the Globe and Mail, the sheer numbers — as well as national media coverage — could have a huge influence on a final decision about the multi-billion project.

U.S. Secretary of State John Kerry is promising a timely decision on the project, as soon as environmental assessments reach his desk within the next few weeks.